1. Field of the Invention
The present invention relates in general to the structure of an electrode used in an oxygen sensor, and more particularly to the electrode structure formed on an oxygen-ion conductive solid electrolyte body of an oxygen sensing element of such an oxygen sensor such that the electrode structure contacts a fluid to be measured, for detecting the concentration of oxygen in the fluid in an accurate manner, even where the fluid contains unburned components or imcombustibles, in particular, oxides of nitrogen (NOx).
2. Discussion of the Prior Art
There is known an oxygen sensor for detecting the concentration of oxygen in exhaust gases (emissions produced as a result of combustion of a fuel) emitted from internal combustion engines for motor vehicles. Such a known oxygen sensor utilizes an oxygen-ion conductive solid electrolyte such as zirconia ceramics, and is operated according to the principle of an oxygen concentration cell In recent motor vehicles, the internal combustion engine is usually controlled so that an air/fuel ratio of an air-fuel mixture to be supplied to the engine is maintained at a desired value For this purpose, the oxygen sensor as indicated above is employed to detect the oxygen concentration of the exhaust emission, which has a given relationship with the air/fuel ratio of the air-fuel mixture. With the oxygen concentration of the exhaust emission detected, the fuel supply to the engine is controlled in a feedback fashion.
The known oxygen sensor or oxygen concentration detecting device of the type described above incorporates an oxygen sensing element wherein two electrodes are disposed on a substrate or body of an oxygen-ion conductive solid electrolyte. One of the two electrodes serves as a measuring electrode which communicates with a fluid to be measured (measurement fluid) such as exhaust gases, while the other electrode serves as a suitable reference electrode which communicates with a reference gas such as an ambient air. In operation, the oxygen sensing element produces n electromotive force, based on a difference in the oxygen concentration between the atmospheres communicating with the measuring and reference electrodes, according to the principle of an oxygen concentration cell. Conventionally, the two electrodes are formed principally of platinum, by plating, sputtering, or other techniques.
However, the known oxygen sensing element using the platinum electrodes as indicated above are not capable of accurately detecting or determining the air/fuel ratio of the air-fuel mixture supplied to the engine, based on the oxygen concentration of the exhaust gases or emissions produced by the engine. Described more specifically, the measuring electrode is not able to sufficiently reduce unburned components, in particular, NOx, contained in the exhaust gases, and is therefore incapable of detecting the compound oxygen in the unburned components. That is, the oxygen sensing element measures the oxygen concentration, without taking the NOx into account. As a result, it is very difficult to control the air/fuel ratio so as to reduce the NOx, based on the detected oxygen concentration, in the known oxygen sensing element.
Various structures of the electrodes have been proposed to improve the durability of the electrodes. For example, laid-open Publication No. 60-144659 (published on July 31, 1985) of Japanese Patent Application No. 59-000039 proposes a technique for maintaining the reliability of a detecting element (oxygen sensing element) for a prolonged period of time, by applying rhodium salt/water or organic solvent to a platinum electrode plated on a solid electrolyte body, and thereby reducing an internal impedance of the element. Further, laid-open Publication No. 61-17950 (published on Jan. 25, 1986) of Japanese Patent Application No. 59-138597 proposes a technique for increasing the life expectancy of a sensing element, by baking a paste of a noble metal applied to the surface of a solid electrolyte body, and forming a metal layer active as a catalyst on the baked noble metal layer, thereby improving an adhering force between the sintered solid electrolyte body and electrode. Another technique for assuring an improved life expectancy of a sensing element is proposed in laid-open Publication No. 61-30760 (published on Feb. 13, 1986) of Japanese Patent Application No. 59-153775, wherein a measuring electrode is covered by a layer of rhodium or palladium or layers of both, to avoid contamination of the electrode by lead contained in the exhaust emissions to be measured.
However, the provision of rhodium or palladium, or catalytic activation species or points on the electrode (platinum electrode) is intended merely for improving the durability of the sensor, but is not improving the detecting characteristic of the sensor, to enhance the property of exhaust emissions produced by motor vehicles, which are controlled based on the output of the sensor.
Thus, the proposed techniques described above do not depart from a technique for pursuing an improvement in the durability of the sensor. More particularly, the techniques disclosed in laid-open Publications Nos. 60-144659 and 61-30760 are nothing but a technique wherein an electrode material applied by plating, deposition, sputtering or other methods is provided with a catalytically active material (rhodium, palladium, etc.). The proposed electrode layer has a small thickness and a small surface area of contact with the solid electrolyte body, and cannot be expected to provide a sufficiently high effect of reducing unburned components contained in the exhaust emissions. Therefore, the sensor utilizing the proposed electrode structure is not able to accurately detect the air/fuel ratio of the air-fuel mixture which produces the exhaust emissions containing NOx. In the technique disclosed in the laid-open Publication No. 61-17950, the metal layer as a catalytic activation layer is formed by applying an aqueous solution of rhodium chloride/palladium chloride to a solid electrolyte body and firing the applied unfired catalytically active material. Accordingly, the thickness of the fired electrode layer is small, and the thickness of the catalytic activation layer is also limited. Hence, the electrode obtained by this technique does not have a sufficiently high effect in reducing the unburned components of the exhaust emissions, and cannot be considered to contribute to an improvement in the detecting accuracy of the sensor, i.e., an improvement in the accuracy of determination of the air/fuel ratio of the air-fuel mixture.
Also, U.S. Pat. No. 4,199,425 discloses a technique wherein a porous ceramic overcoat which covers an electrode is provided with rhodium which serves as a reducing catalyst for promoting chemical equilibrium of oxidizable exhaust gas components on the electrode. Since the catalyst is applied to the overcoat by impregnation or chemical deposition, the amount of ,the catalyst applied to the overcoat is extremely limited. Further, since the catalyst is given to the overcoat, there exists only a comparatively small number of reaction points among the catalyst, platinum and solid electrolyte (zirconia). Therefore, the electrode covered by the overcoat cannot be considered to have sufficiently high ability to reduce the unburned components of the exhaust emissions, and the sensor using the electrode does not permit accurate detection of the air/fuel ratio of the air-fuel mixture which produces the exhaust emissions containing NOx.